This invention relates to cathode ray tubes (CRTS) for display systems, and more particularly relates to such CRTs having an auxiliary magnetic field-producing coil for modifying electron beam scanning to produce display enhancements.
Such coils for display enhancement are known. For example, U.S. Pat. No. 5,291,102, issued to Washburn, relates to such a coil for enhancing the dynamic color separation of a CRT display.
The use of such coils for modulating the scanning velocity of the electron beams is also known.
Such scan velocity modulation (SVM) has been shown to be a very effective and desirable way to increase the apparent resolution and xe2x80x9csparklexe2x80x9d of direct view and projection CRT systems. In operation, changes in electrical current through the SVM coil related to the display signal cause the scanning speed of the electron beams to decrease as the beams traverse boundaries between dark and light areas of the display. This increases the dwell time of the electron beams on the phosphor screen, which is perceived by a viewer as a sharpening of these boundaries, particularly boundaries in the vertical direction.
However, SVM is not universally employed for this purpose due in part to the relatively high cost of adding such a component to the CRT. A large part of this cost is due to the transducer, a small Helmholtz coil that is placed on the neck of the CRT.
The general principles as well as various specific designs of scan velocity modulation (SVM) circuits and transducer coils are known. See for example, U.S. Pat. Nos. 5,093,728 (SVM drive circuitry and system to prevent overheating); U.S. Pat. No. 5,179,320 (coil based on PCB flex circuit design wrapped around neck of CRT); U.S. Pat. No. 5,223,769 (conventional frame and wire coil mounted on neck of CRT); and see European Patent Application 0 592 038 A1 (coil supported by a synthetic resin sleeve mounted on the neck of the CRT).
The design in commercial use at the present time is the flexible coil based on PCB technology, wrapped around the neck of the CRT. This coil is expensive particularly because of the need to meet UL safety rules for smoke and flammability.
Furthermore, despite its flexibility, it is difficult to mount such a PCB coil in the ideal location just ahead of the exit apertures of the electron gun, since such a location corresponds to the steeply curved transition region between the neck and the funnel of the CRT envelope.
The English language abstract of Japanese Patent Application 63-128530 teaches printing each half of an SVM coil on the surface of one of the pair of glass beads which support the electrodes of the electron gun. While this design eliminates the flexible substrate in present use, and moves the coil closer to the electron beams, possibly reducing the power requirements for the coil, the design has several serious drawbacks.
First, the placement of the coils on the glass beads or multiforms, as they are also known, results in the magnetic field being created within the electron gun. This requires sufficient power to overcome the natural magnetic shielding effect of the metal gun parts, and risks disturbing the focusing performance of the gun, particularly the widely used xe2x80x9cin-linexe2x80x9d type of gun.
Second, the relatively long, narrow shape of the multiforms forces the SVM coil halves to also be long and narrow, further sacrificing the efficiency of coil performance.
Third, the outer surfaces of the multiforms are poorly controlled at the present time since they are not critical to the CRT design. Thus, there is considerable variation in surface characteristics such as surface smoothness, from tube to tube, unless additional costs are incurred in producing multiforms with uniform surface characteristics. Without such uniformity, it would be difficult to produce SVM coils with the required characteristics.
Fourth, in order to supply power to the coil, two extra pins would be required in the base of the tube, thus complicating and increasing the cost of manufacture of the tube.
Fifth, the placement of the coil inside the tube means that the tube manufacturer would have to provide the coil, thus preventing the system (eg., television set) manufacturer from purchasing a single, less expensive tube type, and adding the SVM coil only to those tubes destined for more expensive xe2x80x9chigh endxe2x80x9d television sets, such as projection television sets.
Sixth, since the coil is formed inside the tube""s vacuum sealed envelope, the materials and processing used to form the coil must be compatible with the demanding requirements of the tube design and processing; otherwise, the performance and/or life of the tube may be affected. A greater choice of materials and processes is thus available if the coil is placed outside of the tube envelope.
Accordingly, an object of this invention is to provide a CRT with a low cost magnetic field-producing coil such as an SVM coil, which avoids the above disadvantages.
In accordance with the invention, a CRT is provided in which such a coil is formed directly on the envelope of the CRT.
Such a coil is preferably formed in accordance with the invention on an outside surface of the tube""s glass envelope, most preferably in the transition region between the neck and the funnel portions of the envelope.
Such a coil may be formed, for example, by any of several processes suitable for mass production, such as photolithography, silk screening, or printing.
Typically, such a coil is a Helmholtz coil with two halves, each half having from about three to seven turns and a current carrying capacity of about 450 milliamps. This resolution and current carrying capability are well within the capabilities of the these forming processes. For example, a coil formed from a 0.02 inch wide copper strip produced by photolithographic techniques such as are used in the fabrication of printed circuit boards (PCBs) can carry a 1 ampere current with essentially no temperature rise.
When such a scan velocity modulation (SVM) coil is formed directly on the surface of the envelope of a cathode ray tube (CRT) adjacent the exit end of the electron gun, it results in improved efficiency and reduced cost over conventional coils mounted on separate substrates or fixtures attached to the neck. In addition, a uniformity of coil characteristics is obtainable due to the uniformity of the envelope surface on which the coil is formed.
When the coil is formed on the outside surface of the tube envelope, a greater choice of materials and processes is available than if the coil is formed on the inside of the envelope. Moreover, such a coil can be provided by the system manufacturer on selective CRTs, leaving the CRT manufacturer free to produce a limited number of tube types, at higher volume and lower cost.